1. Field of the Invention
The present invention relates to a gear shift control of an automatic transmission of a vehicle, and more particularly, to an improvement of a gear shift control of an automatic transmission for decreasing tire slip of vehicle wheels.
2. Description of the Prior Art
The automatic transmission of vehicles is generally controlled by a gear shift control device so as to be shifted among a plurality of shift stages according to a comparison of engine throttle opening versus vehicle speed such that, when the vehicle is started from standstill and gradually accelerated, the automatic transmission is successively shifted up to a higher shift stage according to a relative increase of the vehicle speed against the throttle opening, or when the vehicle is decelerated from a high speed running state, the automatic transmission is successively shifted down to a lower shift stage according to a relative reduction of the vehicle speed against the throttle opening. In addition to such a regular shift stage change control, it is known to control the shift stage change of the automatic transmission such that the stage is shifted up when the slip ratio of the drive wheel in the direction of decelerating the vehicle becomes excessive, for the purpose of improving the running stability of the vehicle, as described in Japanese Patent Laid-open Publication 6-34038, because the force which would slide the drive wheel along the road surface is the vector addition of a lateral force acting at the drive wheel due to a centrifugal force generated in the turning vehicle and a longitudinal force acting at the drive wheel corresponding to the longitudinal slip which can be decreased by an upshift of the transmission when the slip ratio is oriented in the direction of decelerating the vehicle. The improvement of the turn running stability by such an upshift of the automatic transmission against an excessive increase of the longitudinal slip of the drive wheel will be more effectively obtained when the transmission is shifted up for more stages, if available. However, when the transmission is shifted up by a greater number of shift stages, the drive torque of the drive wheel is reduced by a correspondingly greater ratio, substantially affecting the power performance of the vehicle.